Photosynthesis

What photosynthesis is

Photosynthesis is the process plants (and algae) use to make their own food (glucose) using light energy. It takes place in the chloroplasts, which contain the green pigment chlorophyll that absorbs light. It is an endothermic reaction — energy is taken in from light.

The equation

Word equation:

carbon dioxide + water  →(light)→  glucose + oxygen

Symbol equation:

6CO₂ + 6H₂O  →(light)→  C₆H₁₂O₆ + 6O₂

• Raw materials: carbon dioxide (from the air, via the leaves) and water (from the soil, via the roots).
• Products: glucose (used by the plant) and oxygen (released as a by-product).

What the plant uses glucose for

• Respiration – to release energy.
• Converted to starch for storage (insoluble, so doesn't affect osmosis).
• Making cellulose for cell walls.
• Making proteins (with nitrate ions from the soil).
• Stored as oils/fats (e.g. in seeds).

Limiting factors

The rate of photosynthesis is controlled by whichever factor is in shortest supply — the limiting factor:

Factor	Effect
Light intensity	More light → faster rate (until another factor limits)
Carbon dioxide concentration	More CO₂ → faster rate (until another factor limits)
Temperature	Higher temperature speeds it up — but too high denatures the enzymes and the rate falls

On a graph, the rate rises then levels off when a different factor becomes limiting.

Inverse square law (light)

Light intensity decreases as distance from the plant increases. If you double the distance, the light intensity drops to about a quarter. In experiments, moving a lamp closer increases the rate of photosynthesis (more bubbles of oxygen from pondweed).

Required practical

Investigating the effect of light intensity on photosynthesis using pondweed: count the oxygen bubbles (or measure gas volume) produced per minute at different distances from a lamp. Closer lamp = more light = more bubbles.

Worked example

A plant's photosynthesis rate rises as light increases, then flattens. It's a warm, sunny day. What is now limiting it?

• Light is plentiful and it's warm, so the rate has plateaued — the limiting factor is most likely carbon dioxide concentration. ✓

Common mistakes

• Saying photosynthesis "makes energy" — it stores light energy as chemical energy in glucose.
• Forgetting it is endothermic (takes energy in).
• Only naming light as a limiting factor — remember CO₂ and temperature too.

Exam tips

• Learn both the word and symbol equations.
• For graph questions, identify which factor is limiting at each stage (flat part = something else is limiting).
• Link high temperature falls to enzymes denaturing.

Key facts to remember

• CO₂ + water →(light, chlorophyll)→ glucose + oxygen; happens in chloroplasts; endothermic.
• Glucose is used for respiration, starch, cellulose, proteins and oils.
• Limiting factors: light intensity, CO₂ concentration and temperature (too hot denatures enzymes).